Retrolisthesis is a spinal condition involving the backward displacement of a single vertebra relative to the one immediately below it. This misalignment can occur anywhere along the spine, but is most commonly observed in the neck (cervical spine) and lower back (lumbar spine). When this slippage happens specifically at the C5-C6 level, it affects a segment of the neck responsible for significant movement and stress absorption. The posterior shift of the C5 vertebra on the C6 vertebra can narrow the space available for the delicate neural structures, leading to a variety of symptoms.
Anatomy of the C5 C6 Vertebrae
The cervical spine consists of seven vertebrae (C1 through C7) that support the head and allow for a wide range of motion. The C5 and C6 vertebrae form a critical spinal motion segment in the lower part of the neck. This segment includes the two vertebral bodies, the intervening intervertebral disc, and a pair of facet joints at the back.
The intervertebral disc acts as a shock absorber, while the facet joints guide neck movement. The C5-C6 segment is a frequent site of degenerative changes because it endures considerable mechanical stress from supporting the head and facilitating neck flexion and extension. The spinal cord runs through the central canal, and the C6 spinal nerve root exits just below the C5 vertebra.
Defining Retrolisthesis at C5 C6
Retrolisthesis at C5-C6 is defined as the backward displacement of the C5 vertebral body with respect to the C6 vertebral body below it. This is the opposite of spondylolisthesis, where the vertebra slips forward.
The primary mechanisms leading to this slippage are usually related to degenerative changes. Severe disc degeneration, where the intervertebral disc loses height and stability, is a common cause, as is arthritis in the facet joints. As the disc shrinks, the stabilizing ligaments can become lax, allowing the upper vertebra to slide backward. Traumatic injury, such as whiplash or a significant fall, can also cause acute retrolisthesis by damaging the ligaments that hold the vertebrae in place.
The severity of the displacement is typically graded based on the percentage of the C5 vertebral body that has slipped backward over the C6 body. A Grade 1 retrolisthesis involves up to 25% slippage, while higher grades, such as Grade 2 (25% to 50%), indicate more significant instability.
Specific Symptoms and Neurological Impact
The symptoms of C5-C6 retrolisthesis relate directly to the compression of neural structures. When the C5 vertebra shifts backward, it can narrow the space for the spinal cord and the exiting C6 nerve root. Compression of the C6 nerve root causes radiculopathy, characterized by pain, numbness, or tingling that radiates down the arm.
The C6 radiculopathy pattern typically involves pain and sensory changes that travel down the arm, often extending into the thumb and index finger. Patients may also experience muscle weakness, specifically affecting the wrist extensors and the biceps muscle, and a diminished reflex in the brachioradialis tendon.
A more severe complication is cervical myelopathy, which occurs when the spinal cord itself is compressed by the shifted vertebra or associated structures like bone spurs. Myelopathy symptoms are usually less localized than radiculopathy and can affect the entire body below the level of compression. Early signs often include difficulty with fine motor skills, such as buttoning a shirt or writing, along with problems in balance and gait instability.
Diagnosis and Management Options
Diagnosis begins with a physical and neurological examination to assess reflexes, muscle strength, and sensory changes. Imaging is then used to confirm the diagnosis and determine the extent of the slippage. Standard lateral X-rays of the neck are the initial tool for visualizing the vertebral alignment and measuring the degree of posterior displacement.
To assess spinal stability, doctors may also order flexion and extension X-rays, which capture the movement of the vertebrae as the patient gently bends their neck forward and backward. An MRI is often used to get a detailed view of the soft tissues, including the intervertebral discs, ligaments, and any compression of the spinal cord or nerve roots.
Management starts with conservative treatments for most low-grade cases. These non-surgical options include physical therapy to strengthen the neck and core muscles, nonsteroidal anti-inflammatory drugs (NSAIDs) for pain, and sometimes a soft cervical collar for temporary support. Epidural steroid injections may be administered to reduce inflammation around the compressed nerve root.
Surgical intervention is considered when conservative measures fail or if there is evidence of progressive neurological deficit or significant spinal instability. Procedures generally focus on decompressing the affected nerve structures and stabilizing the segment, often involving a fusion procedure where the C5 and C6 vertebrae are permanently joined.